The invention relates in general to a voltage boost circuit, and more particularly, to a voltage boost circuit having a single inductor. The voltage boost circuit provides a regulated constant first output at a potential above the supply voltage and a regulated constant second output.
Various voltage boosting systems are known for outputting constant raised voltages from fluctuating input voltages. Such systems have been used in automotive electronics applications. For the past several years, the number of electronics devices in automobiles has increased rapidly as vehicles have become more sophisticated. In many instances electronic devices that were not available several years ago are standard equipment in current automobiles. Much of this new equipment is computer controlled and requires energization from regulated constant voltage.
Automobiles have "body computers" for controlling items such as lamps, automatic door locks, windows, and the like. Computers are also necessary for many dashboard components, such as trip computers and other digital display devices. It is known, however, that the majority of these devices requires their own particular voltage levels for proper operation.
Typically, motor vehicles utilize a 12-volt storage batteries while providing battery voltages between 9 volts and 12.5 volts. However, automobile batteries are generally unable to provide constant voltages during different operating conditions. Large current draw devices, such as the starter, may cause the battery voltage to drop as low as 4.5 volts during a cold-crank start. The battery voltage may range as high as 35 volts during other transient conditions. Because of these wide voltage fluctuations such unregulated battery power is unsuitable for the voltage critical applications as described above.
A known voltage boost circuit comprises an inductor coupled to receive an input battery voltage which causes an inductor current to flow. From time to time the inductor current is interrupted by a switch thereby inducing an inductor voltage greater than the input battery voltage. Current pulses resulting from the switching incrementally charge a capacitor to a desired output voltage, at which point the switching is suspended. As current is drawn from the capacitor further inductor current pulses are generated by operating the switch at a fixed frequency, which generally allows each pulse of inductor current to decay to zero before the switch is closed.
In operation, the pulses are generated using a 50% duty cycle internal pulse generator that is enabled when the battery potential goes below a certain voltage level, typically 9 volts. At that point, the inductor is modulated at 50% so that the output voltage is regulated at 9 volts or is boosted to a higher voltage. Unfortunately, such known voltage boost circuits have relatively low switching frequencies. This results in the need for relatively very large and expensive inductors having inductance in the range of approximately 300 microhenries to 400 microhenries, to provide the appropriate current and voltage levels and to prevent the output current from decaying to zero before the switch switches and the current can be built back up. Therefore such large inductors are necessary to maintain the appropriate amount of current flow to the output for an extended period of time between switching in order to prevent the capacitor output voltage from dropping below its required level. A further disadvantage of these prior voltage boost circuits is that their output voltages are regulated by "dropping" current-providing cycles in order to avoid exceeding a preselected output voltage. The cycles are dropped by stopping switch cycling. This causes a quasi-DC current to flow through the inductor. The quasi-DC current is limited primarily by the internal resistance of the battery and the resistance of the inductor itself. The result is a large current draw which wastes current and may overheat the circuit. However, as can be seen, this results in a waste of current and a voltage boost circuit that generates excessive heat from constantly being on. A further disadvantage in these prior voltage boost circuits is that while multiple outputs are available, the voltage boost circuits are unable to provide different amounts of voltage on each output and accordingly also do not automatically vary the amounts of current provided to each of the outputs.
Prior voltage boost circuits also have the disadvantage of using voltage comparators to enable or disable the boost circuitry. As such, because of the absence of hysteresis on these voltage comparators, a significant amount of noise from electromagnetic interference (EMI) is generated when the boost circuitry is switched on and off. The noise is generated by the jitter caused in the circuit due to the fairly rapid switching of the voltage boost circuit, and may in turn cause noise to be heard on the car radio and other radio frequency type devices.